The role of on-site trips in recreation demand modeling is examined by means of the travel cost method. An understanding of on-site choices of site attractions and costs incurred by visitors is important if analysts are to estimate the popularity of the various attractions and, ultimately, to derive site benefits. We investigate two primary questions. First, can on-site trips to attractions be adequately modeled? Next, how important is onsite travel in valuing site benefits? Visitor survey data for the North Carolina Zoological Park were used to determine that economic site benefits are underestimated by 24% if expected on-site travel costs from recreation demand analysis are excluded.